A Manual of Elementary Geology

- Wood bored by Teredina - Tripoli and semi-opal formed of infusoria - Chalk derived principally from organic bodies - Distinction of freshwater from marine formations - Gene

stratified deposits. We should often be unable to detect any signs of stratification or of successive deposition, if particular kinds of fossils did not occur here and there at certain depths in the mass. At one level, for exam

he last chapter, and allows sufficient time for the accumulation of sediment. He must never lose sight of the fact that, during the process of deposition, each separate layer was once the uppermost, and covered immediately by the water in w

yte has been determined by the manner in which it grew originally. The axis of the coral, for example, if its natural growth is erect, still remains at right angles to the plane of stratification. If the stratum be now horizontal, the round spherical heads of certain species continue uppermost, and their points of attachment are directed downwards. This arrangement is sometimes repea

g.

oth on the outside and in

n than to see fossil oysters in clay, with serpul?, or barnacles (acorn-shells), or corals, and other creatures, attached to the inside of the valves, so that the mollusk was certainly not buried in argillaceous mud the moment it died. There must have been an interval during which it was still surrounded with clear water, when the testacea, now adhering to it, grew from an embryo state to full maturity. Attached shells which are merely external, like some

ell known that these animals, when living, are invariably covered with numerous spines, which serve as organs of motion, and are supported by rows of tubercles, which last are only seen after the death of the sea-urchin, when the spines have dropped off. In fig. 12. a living species of Spatangus, common on our coast, is represented with one half of its

g.

o a fossil Spatang

g.

ith the spines rem

d tubercles

same ma

lusca. The upper valve (b, fig. 13.) is almost invariably wanting, though occasionally found in a perfect state of preservation in white chalk at some distance. In this case, we see clearly that the sea-urchin first lived from youth to age, t

g.

alk, with lower valve

ve of the Cr

ecent wood pierced by the Teredo navalis, or common ship-worm, which destroys wooden piles and ships. When the cylindrical tube d has been extracted from the wood, a shell is seen at the larger extremity, composed of two pieces, as shown at c. In like manner, a piece of fossil wood (a, fig. 14.) has been perforated by an animal of a kindred but extinct genus, called Teredina by Lamarck. The calcareous tube of this mollusk was united and as it were soldered on to the valves of the shell (b), which therefore cannot be detached from the tube, like the valves of the recent

wood drilled by pe

ood from London clay

sonata, the right-hand figure the

ecent wood bo

of Teredo navali

view of the valves of sa

n, that a certain kind of siliceous stone, called tripoli, was entirely composed of millions of the remains of organic beings, which the Prussian naturalist refers to microscopic Infusoria, but which most others now believe to be plants. They abound in freshwater lakes and ponds in England and other countries, and are termed Diatomace? by those naturalists who believe in their vegetable origin. The substance alluded to has long been well known in the arts, being used in the form of powder for polishing stones and metals. It has been procured, among other places, from Bilin, in Bohemia, where a single stratum, extending over a wide area, is no les

Bacillari

Gaillonel

aillonella

s, except the lower figure of G. ferruginea

al from the great b

. Natur

ircular articulations of a species of

he same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spicul? or internal supports of the freshwater sponge, or Spongilla of Lamarck, are sometimes intermingled (see the ne

and spicul? of the Spongilla are filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomace? have been dissolved by water, and have thus given rise to this opal i

, as termed by Ehrenberg) are freshwater; but in other countries, as in the tripoli of the Isle of France, they a

consist of innumerable articulated threads, of a yellow ochre colour, composed partly of flint and partly of o

Foraminifera f

ere, Müll. Cy

ortion of

ristellari

4. Ros

tions of white chalk from different parts of England, found, on carefully pulverizing them in water, that what appear to the eye simply as white grains were, in fact, well preserved fossils. He obtained above a thousand of these from each pound weight of chalk, some being fragments of minute corallines, others entire Foraminifera and Cytherid?. The annexed drawings will give an idea of the beautiful forms of many of these bodies. The figures a a represent their natural size, but, minute as they seem, the smallest of them, such as a, fig. 24., are gigantic in comparison with the cases of Diatomace? before mentioned. It has, moreover, been lately discovered th

ead upon was on

ous and siliceous dust of which hills are composed has not only been once alive, but almost every particle, albeit invisible to the na

t lakes and rivers are distinct from those inhabiting the sea. In the northern part of the Isle of Wight a formation of marl and limestone, more than 50 feet thick, occurs, in which the shells are principa

or instance, the limestone on the left bank of the Rhine, between Mayence and Worms, at Oppenheim, Findheim, Budenheim, and other places. In order to account for this phenomenon, the geologist has only to examine the small deltas of torrents which enter the Swiss lakes when the waters are low, such as the newly-formed plain where the Kander enters the Lake of Thun. He there sees sand and mud strewed over with innumerable dead land shells, which have been

re usually very limited in area when compared to marine deposits, just

s of the mollusca that we are guided in determining the point in question. In a freshwater deposit, the number of individual shells is often as great, if not greater, than in a marine stratum; but there is a smaller variety of species and genera. This might be anticipated from the fact that the genera and species of r

g.

vata; foss

g.

rina; fossil.

g.

rdierii; fo

g.

marginatus; r

g.

alis; recen

ng freshwater. Among these last, the four most common forms, both recent and fossil, are Cyclas, Cyrena, Unio,

g.

w. (G. arcuata, Lam

ig. 25., and the Monomyary, such as the oyster and scallop, in which there is only one of these impressions, as is seen in fig. 30. Now, as

g.

phalus; fossil

g.

iscata; fos

g.

enta; foss

Lymnea, and Paludina. (See figures.) But to these are occasionally added Phys

g.

ibia; fossil.

g.

egans; fos

g.

ossil. Gra

g.

ypnorum

g.

a; rece

g.

quinata. P

g.

mnaris. Pa

g.

uccinoidea;

it sometimes differs in no respect from the marine Siphonaria, except in

g.

lobulus. P

g.

nulosa. Pa

stinguish the two by good generic characters. But, as a general rule, the fluviatile species are smaller, smoother, and more glob

g.

cinctum. P

both to rivers and the sea, having species peculiar to each. Other genera, like

g.

ensis. Falu

g.

ma elega

g.

ridens

g.

a bidens

g.

bricus. Lo

and fossil state, are Helix (fig. 45.), Cyclostoma (fig. 46.), Pupa (fig. 47.), Clausilia (fig. 4

g.

glauca, fro

s. Many fossil species have been referred to this genus, but they have been found chiefly in mari

distinguishing freshwater from marine strata; since, if any univalves occur of which the mouths are not entire, we may presume that the formation is marine. The aperture is said to be entire in such shells as the Ampullaria

g.

tata. Subap.

g.

subulata. L

and almost all such species are carnivorous; whereas nearly all testacea having entir

The Cerithium (fig. 44.), although provided with a short canal, comprises some species which i

genus swarm in lakes and stagnant pools in Great Britain; but their shells are not, if considered separately, conclusive as to the freshwater origin of a deposit, because the majority of species in another ki

n freshwater strata. These seed-vessels were called, before their true nature was k

ure (fig. 54.) represents a branch of one of many new species found by Professor Amici in the lakes of northern Italy. The seed-vessel in this plant is more globular than in the British Char?, and therefore more nearly resembles in form the extinct fossil species found in England, France, and other countr

g.

nula; fossil.

l. magnified

m, mag

g.

tica; rece

ween the division of the l

ch, with five seed-vessels magn

r shells; and we also find occasionally the teeth and bones of land quadrupeds, of species now unknown. The manner in which such remains

me marine species, as Cottus, Mugil, and Anguilla, or eel. The rest are either common to rivers and the sea, as the salmon; or are exclusively characteristic of salt water. The above observations respecting fossil fishes are applicable only to the more modern or tertiary

arisen from the alternate occupation of certain spaces by river water and the sea; for in the flood season the river forces back the ocean and freshens it over a large

the sea, and which are filled with salt and fresh water by turns. They often communicate exclusively with the river for month

gs, has been four times fresh and four times salt, a bar of sand between it and the ocean having been as often formed and removed. The last irruption of salt water happened in 1824, when the North Sea entered, killing a

resting on freshwater strata. When we find, as in the south-east of England, a great series of freshwater beds, 1000 feet in thickness, resting upon marine formations and again cov